Differential amplifier configurations which each include at least one pair of differentially connected transistors have been advantageously employed in electronic equipment. One advantage of the differential amplifier is that under ideal conditions common-mode input signals do not affect the output signal. More specifically, any signal which causes the inputs of both differentially connected transistors to increase or decrease equally and simultaneously will have no deleterious effect on the differential voltage developed between the collectors of the differentially connected transistors. Therefore, equal changes in the collector-junction leakage currents or in the base-to-emitter voltages with temperature variation of the differential transistors do not affect the magnitude of the differential voltage between the collectors provided that the differentially connected transistors have matched characteristics. Closely matched transistors of the same conductivity type are achievable in monolithic circuits because of the simultaneous processing of components on each wafer. Also, critical components can be placed in close spacial proximity to assure substantially equal temperatures. Hence, undesirable balanced input signals, such as thermally induced voltage and current changes, that are applied equally to each of the monolithic differential transistors are balanced out rather than being passed on to either subsequent stages of the circuit or to the load.
Differential amplifiers having balanced driving circuits are suitable for amplifying input signals from symmetrical signal sources such as strain-gauges, bridge circuits and balanced transmission lines, each of which provide a balanced signal which is not directly referenced to ground. Unbalanced differential amplifiers are suitable for amplifying input signals which are referenced to ground. Regardless of whether the differential amplifier is driven by balanced or unbalanced signal supplies, the output signals developed between the collectors of the differentially connected transistors of the differential amplifier are balanced. Hence, the output signals of differential amplifiers may be directly fed only to a balanced load such as another balanced amplifier. In many applications, it is desired to transform the inherently balanced or double-ended output signal of the differential amplifier into an unbalanced or single ended output signal.
Prior art "turn-around" circuit configurations for providing double-to-single ended or balanced-to-unbalanced conversion sometimes tend to unbalance the differential amplifier itself and thereby destroy many of its inherently desirable characteristics. Also, some prior art turn-around circuit configurations create still other problems. More specifically, one prior art turn-around or double-to-single ended converter stage includes a diode connected transistor which is connected to the collector of one of the differential transistors and another transistor having a base connected to the base of the diode connected transistor and a collector connected both to the collector of the other differential transistor and to the input terminal of an emitter follower amplifier including two transistors. If this turn around configuration is provided in an economical, monolithic integrated circuit form, then the betas or D.C. current gains of the transistors included therein are relatively low. As a result, the difference in base currents demanded from each of the differential transistors by the turn-around circuit during quiescent operation tends to unbalance the differential amplifier. In particular, one of the differential transistors is required to provide base current to two transistors while the other of the differentially connected transistors is required to provide base current to only one transistor. Moreover, the magnitudes of the base currents required by each of the turn-around transistors may be different because of their differing quiescent collector currents.
Consequently, an undesirable "input offset voltage" results across the input terminals of the differential amplifier. "Input-offset voltage" is defined as the difference in base-to-emitter input potentials required for equal emitter or collector current in the differential transistors. The resulting current mismatch in the differential transistors, which changes as a function of temperature, causes the input-offset voltage to also vary as a function of temperature.
Another disadvantage associated with the above described prior art circuit is that it creates a quiescent voltage at the collector of one of the differential transistors of one "V.sub.be ", which is defined as the base-to-emitter voltage of a transistor, and a voltage of 2V.sub.be at the collector of the other differential transistor. Thus, the maximum amplitude of the dynamic input common mode voltage of the differential amplifier is undesirably limited as a result of the 2V.sub.be collector voltage.
To solve the limited dynamic range problem of the foregoing prior art circuit, another prior art turn-around circuit has been developed in which a transistor of opposite conductivity type is connected between the emitter follower amplifier and the remainder of the turn-around circuit. Since most present day monolithic circuits are made in N-epitaxial type material, this transistor of opposite conductivity type is of a PNP conductivity. The V.sub.be of the PNP transistor increases the maximum magnitude of the common mode dynamic signal by cancelling the effect of the V.sub.be of one of the emitter-follower transistors.
Unfortunately, however, the quiescent base current from the PNP transistor tends to further unbalance the collector currents of the differentially connected transistors, unless the beta or D.C. current gain of the PNP transistor is very high. In inexpensive monolithic structures, PNP transistors are normally provided in lateral form. Because of the surface imperfections in such structures, such lateral PNP transistors generally have low betas. Although vertical PNP transistors having high betas can be manufactured using dielectrically isolated monolithic structures, the expense of this process is generally prohibitive. Thus, utilization of low beta PNP transistors in the prior art turn-around circuit to increase the dynamic swing has the adverse effect of further unbalancing the magnitudes of quiescent currents at the collectors of the differentially connected transistors. Consequently, the input offset voltage of such circuits is undesirably increased. Since the input offset voltage is generally monitored during final test of monolithic amplifiers, this increase in input offset voltage causes a decrease in yield and thus an overall increase in the cost of the completed integrated circuit.